Java Basics : Comparable vs Comparator

Java Basics : Comparable vs Comparator

Difference

Comparable lets a class implement its own comparison:

• it's in the same class (it is often an advantage)
• there can be only one implementation (so you can't use that if you want two different cases)
• A comparable object is capable of comparing itself with another object. The class itself must implements the java.lang.Comparable interface in order to be able to compare its instances.
• Comparable provides compareTo() method to sort elements.
• Comparable is found in java.lang package
• We can sort the list elements of Comparable type byCollections.sort(List) method.

By comparison, Comparator is an external comparison:

• it is typically in a unique instance (either in the same class or in another place)
• you name each implementation with the way you want to sort things
• you can provide comparators for classes that you do not control
• the implementation is usable even if the first object is null
• A comparator object is capable of comparing two different objects. The class is not comparing its instances, but some other class’s instances. This comparator class must implement the java.util.Comparator interface
• Comparator provides compare() method to sort elements
• Comparator is found in java.util package.
• We can sort the list elements of Comparator type byCollections.sort(List,Comparator) method.

Comparable Example

The Employee class where we are implementing Comparable

===========================================

public class Employee implements Comparable<Employee> {

String empName;

int empId;

public Employee(String empName, int empId )

{ super();

this.empId=empId;

this.empName=empName;

}

@Override

public int compareTo(Employee emp) {

return (this.empId<emp.empId)?-1 :(this.empId>emp.empId)?1:0;

}

public String getEmpName() {

return empName;

}

public void setEmpName(String empName) {

this.empName = empName;

}

public int getEmpId() {

return empId;

}

public void setEmpId(int empId) {

this.empId = empId;

}

}

=================================

The main Class

=================================

import java.util.*;

public class EmployeeComparableMain {

public static void main(String[] args) {

Employee e1= new Employee("Steve",1);

Employee e2= new Employee("Jack",2);

Employee e3= new Employee("Ram",5);

Employee e4= new Employee("Lova",3);

List<Employee> list = new ArrayList<Employee>();

list.add(e1);

list.add(e2);

list.add(e3);

list.add(e4);

Iterator itr=list.iterator();

while(itr.hasNext())

{

Employee emp=  (Employee) (itr.next());

System.out.println("Name :" +emp.getEmpName()+"  Id=" +emp.empId );

}

Collections.sort(list);

itr=list.iterator();

while(itr.hasNext())

{

Employee emp=  (Employee) (itr.next());

System.out.println("Name :" +emp.getEmpName()+"  Id=" +emp.empId );

}

Collections.sort(list);

}

}

============

o/P

============

Name :Steve  Id=1

Name :Jack  Id=2

Name :Ram  Id=5

Name :Lova  Id=3

Name :Steve  Id=1

Name :Jack  Id=2

Name :Lova  Id=3

Name :Ram  Id=5

=================================================

For Comparator

We will create class country having attribute id and name and will create another classCountrySortByIdComparator which will implement Comparator interface and implement a compare method to sort collection of country object by id and we will also see how to use anonymous comparator.

1.Country.java 

public class Country{
    int countryId;
    String countryName;
    
    public Country(int countryId, String countryName) {
        super();
        this.countryId = countryId;
        this.countryName = countryName;
    }

    public int getCountryId() {
        return countryId;
    }


    public void setCountryId(int countryId) {
        this.countryId = countryId;
    }


    public String getCountryName() {
        return countryName;
    }


    public void setCountryName(String countryName) {
        this.countryName = countryName;
    }
    
} 

2.CountrySortbyIdComparator.java

package org.arpit.javapostsforlearning;

import java.util.Comparator;
//If country1.getCountryId()<country2.getCountryId():then compare method will return -1
//If country1.getCountryId()>country2.getCountryId():then compare method will return 1
//If country1.getCountryId()==country2.getCountryId():then compare method will return 0
 public class CountrySortByIdComparator implements Comparator<Country>{

    @Override
    public int compare(Country country1, Country country2) {
        
        return (country1.getCountryId() < country2.getCountryId() ) ? -1: (country1.getCountryId() > country2.getCountryId() ) ? 1:0 ;
    }

}

3.ComparatorMain.java

package org.arpit.javapostsforlearning;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class ComparatorMain {

 /**
  * @author Arpit Mandliya
  */
 public static void main(String[] args) {
   Country indiaCountry=new Country(1, "India");
   Country chinaCountry=new Country(4, "China");
   Country nepalCountry=new Country(3, "Nepal");
   Country bhutanCountry=new Country(2, "Bhutan");
   
         List<Country> listOfCountries = new ArrayList<Country>();
         listOfCountries.add(indiaCountry);
         listOfCountries.add(chinaCountry);
         listOfCountries.add(nepalCountry);
         listOfCountries.add(bhutanCountry);
  
         System.out.println("Before Sort by id : ");
         for (int i = 0; i < listOfCountries.size(); i++) {
    Country country=(Country) listOfCountries.get(i);
    System.out.println("Country Id: "+country.getCountryId()+"||"+"Country name: "+country.getCountryName());
   }
         Collections.sort(listOfCountries,new CountrySortByIdComparator());
         
         System.out.println("After Sort by id: ");
         for (int i = 0; i < listOfCountries.size(); i++) {
    Country country=(Country) listOfCountries.get(i);
    System.out.println("Country Id: "+country.getCountryId()+"|| "+"Country name: "+country.getCountryName());
   }
         
         //Sort by countryName
         Collections.sort(listOfCountries,new Comparator<Country>() {

    @Override
    public int compare(Country o1, Country o2) {
     return o1.getCountryName().compareTo(o2.getCountryName());
    }
   });
   
   System.out.println("After Sort by name: ");
         for (int i = 0; i < listOfCountries.size(); i++) {
    Country country=(Country) listOfCountries.get(i);
    System.out.println("Country Id: "+country.getCountryId()+"|| "+"Country name: "+country.getCountryName());
   }
 }

}

Output:

Before Sort by id : 
Country Id: 1||Country name: India
Country Id: 4||Country name: China
Country Id: 3||Country name: Nepal
Country Id: 2||Country name: Bhutan
After Sort by id: 
Country Id: 1|| Country name: India
Country Id: 2|| Country name: Bhutan
Country Id: 3|| Country name: Nepal
Country Id: 4|| Country name: China
After Sort by name: 
Country Id: 2|| Country name: Bhutan
Country Id: 4|| Country name: China
Country Id: 1|| Country name: India
Country Id: 3|| Country name: Nepal